Why does corrosion occur easily in areas where the medium of the device undergoes phase change? I would appreciate some guidance from the teacher.

This is a good question. First of all, the phase transition mentioned here refers to the vapor-liquid phase transition. When the liquid phase turns into a vapor phase, some components that were originally dissolved in the liquid phase do not vaporize along with the main portion of the liquid due to their lower volatility; as a result, their concentration in the remaining liquid increases. When this concentration exceeds their corresponding solubility limit, these components may even precipitate out. If such components are corrosive, then the phase transition **increases their concentration at the site of phase change, thereby enhancing corrosion there. A common example is the top of the desorption tower in hydrodesulfurization units, where acid water is carried back into the tower by reflux. When the vapor phase turns into a liquid phase, if highly corrosive substances are present in the vapor and can dissolve easily in the liquid, the high concentration of these substances in the liquid leads to an accelerated rate of corrosion. Another common example is the condensation of water in the pipelines at the top of atmospheric pressure towers; this is why additives are used at the top of such towers. There are many similar situations, but I won’t list them all here.

Since the gas itself does not produce hydrogen ions, then how can there be any talk of corrosion? Taking atmospheric and vacuum distillation as an example, excessively high temperatures cause the salts in crude oil to hydrolyze, producing hydrogen chloride and hydrogen sulfide. At the top of the tower, as the steam condenses into water, hydrogen chloride and hydrogen sulfide dissolve naturally in the water, generating hydrogen ions that lead to corrosion. The same principle applies to the rest.

It is not the areas where the medium undergoes phase change that are prone to corrosion; rather, when the gas phase transforms into a liquid phase, corrosion shifts from being primarily physical in nature to being primarily chemical in nature. Chemical corrosion is more severe than physical corrosion, and all chemical corrosion reactions are ionic reactions. Once the gas phase turns into a liquid phase, these ionic reactions intensify, which can result in more severe corrosion. However, it is not enough to consider only the temperatures of the gas-phase and liquid-phase media; the corrosion caused by high-temperature gas-phase media can be greater than that caused by overheated liquid-phase media. Factors such as the material composition of the equipment also need to be taken into account.

As the processing raw materials become more varied and of lower quality, atmospheric and vacuum distillation units must implement anti-corrosion measures such as \"one removal and four injections,\" and ensure the proper operation of electrodialysis desalination facilities. Because the proper or improper operation of electrodesalination facilities affects not only atmospheric and vacuum distillation units, but more importantly, it has a significant impact on the catalysts in catalytic units. Corrosion-prone areas can be treated with corrosion inhibitors, and monitoring of iron ions should be strengthened.

Corrosion is the most troublesome issue for oil refining companies. I learned a lot here today; thank you to all the poster owners

It’s described in very complicated terms. In my opinion, the reason why corrosion of our equipment is more severe in the phase transition area is that a water-based corrosion system tends to form at this time. For example, the tower top condensation cooling system.

I think it’s at the phase transition point where gas is generated, causing the relative pressure to increase; at the same time, the concentration of the liquid also rises, which may lead to more severe corrosion. For the atmospheric pressure tower top: I think the main issue is sulfur corrosion (H2S); I haven’t thought of anything else yet.